Estimating biological N 2 fixation by a tropical legume tree using the non-nodulating phenophase as the reference in the 15N natural abundance method

Abstract

The non-nodulating phenophase of a legume tree was tested as a non-N 2-fixing reference in application of the 15N natural abundance method for estimating the N 2 fixation. We applied this method to study the effects of three pruning intensities, complete pruning every 6 months (T-6), ca. 50% pruning every 3 months (P-3) and intact control (C), on N 2 fixation in Erythrina lanceolata (Papilionaceae) planted as shade and support trees for vanilla ( Vanilla planifolia) in a subhumid tropical site in Quepos, Costa Rica. We measured nodulation and N 2 fixation for 12 months. The trees under the C regime nodulated abundantly during the rainy season vegetative growth but did not nodulate during the dry season and flowering. A linear regression ( r 2=0.76) was observed between the nodule biomass and δ 15 N values under the C regime, suggesting quite a stable specific N 2 fixation rate. The stable δ 15 N values throughout the year in the non-N 2-fixing Morus nigra (Moraceae) growing on the same soil indicated that the temporal variation in the plant available soil 15N was low. We used the intercept term of the regression (δ 15 N=3.5 when nodule biomass was 0) as the non-N 2-fixing δ 15 N value when estimating the percentage of N fixed from atmosphere out of tree total N. The percentage varied from 0 during the driest period to 53% during rainy season. Pruning reduced rainy season nodulation under the T-6 and P-3 regimes almost to nil, and the δ 15 N values were high. Our results suggest that the conditions for using the non-nodulating phenophase as the non-N 2-fixing reference required in the 15N natural abundance method were fulfilled. The C trees showed a clear phenological cycle in N 2 fixation, while pruning severely disturbed the N 2 fixation under the T-6 and P-3 regimes, indicating that E. lanceolata is better suited for agroforestry systems in which the trees are not managed by periodic prunings.